Current semiconductor devices, including processors, memory storage devices, controllers, and other integrated circuits, include transistors. A transistor is a three node device that controls the flow of electric current between two of its nodes, called the source/drain regions, based on the amount of charge applied to a third node, called the gate. Semiconductor devices are being manufactured in increasingly smaller sizes to increase their speed and decrease their power consumption and manufacturing cost. Unfortunately, as transistor size is reduced, the transistor gates become more susceptible to damage. A damaged transistor gate can result in either a poor quality semiconductor device or a nonfunctional device.
The gates of most transistors include three components, metal, oxide, and semiconductor material. For this reason, these transistors are called MOS transistors. The metal portion of the gate is also known as the gate electrode. The gate electrode may include metal or any other electrically conductive material such as highly doped polysilicon. As used herein, the term "gate" refers to the gate electrode. The gate oxide may include silicon dioxide or any other dielectric material such as silicon nitride. The semiconductor material may include silicon or any other semiconductor material such as gallium-arsenide.
One way that transistor gates can be damaged is by gate charging. Gate charge damage occurs when the gate of a transistor becomes charged (positively or negatively). Once this charge exceeds a certain threshold, the gate oxide breaks down, and the gate is discharged through the gate oxide and into the semiconductor substrate. This can damage the gate oxide, causing the transistor to perform poorly or to be entirely nonfunctional. Charging can also damage other structures in a semiconductor device, such as capacitors, diodes, resistors, and electrical interconnects.
A transistor gate is charged up during the manufacturing process. During processing of the semiconductor device, electrical interconnects, primarily comprising aluminum, are etched using an etch technique called reactive ion etching (RIE). In RIE, high voltages are applied to a gaseous mixture of etching chemicals in a vacuum chamber to form a plasma. The semiconductor device resides inside the chamber on a plate that is electrically biased with respect to a plate disposed on the opposite side of the plasma. The bias causes energetic ions from the plasma to accelerate toward the semiconductor device. Upon contacting the surface of the semiconductor device, the ions react with elements in the electrical interconnects, such as aluminum, to form a volatile compound that is pumped out of the chamber.
The high voltages and ionic reactions during RIE processing result in the electrical interconnects becoming charged up. Some of these electrical interconnects are coupled to transistor gates. Because of the conductive nature of electrical interconnects, charge on the electrical interconnects is transferred to the transistor gates. When the charge on the electrical interconnects becomes too large (either positively or negatively), the gate oxide breaks down, damaging the transistor.
One way to prevent this breakdown during processing is to couple a protection circuit to the electrical interconnects that are coupled to transistor gates and other structures in the semiconductor device that are susceptible to charge damage. A protection circuit is simply a diode, coupled to the interconnect, that is reverse biased when the interconnect is charged up. The diode is designed such that its reverse bias breakdown voltage is less than the voltage at which the protected transistor gate oxide breaks down.
Unfortunately, as transistors become smaller and gate oxide thickness decreases, the voltage at which the gate oxide breaks down may become smaller than the reverse bias breakdown voltage of the diode in the protection circuit. In this scenario, the protected structure is damaged before the protection circuit is activated.